Conventionally, when an agricultural tractor, a passenger vehicle or the like is running, the wheels and engine are interconnected through a reduction system. When the vehicle travels down an inclined terrain, the engine brake is operable to suppress acceleration of the vehicle by gravity.
FIG. 5 shows a commonly known transmission for a vehicle. This transmission comprises a propelling line for transmitting power from an engine 1 to a running device 3 through a hydraulic clutch 31 and a first transmission gearing 32, and a propelling line for transmitting power from the engine 1 to the running device 3 through a second transmission gearing 33, a one-way clutch 34 and a transmission shaft 35. With this transmission, when the hydraulic clutch 31 is disengaged, the power is changed into a plurality of speeds by the second transmission gearing 33 and transmitted to the running device 3 through the transmission shaft 35. When the hydraulic clutch 31 is engaged, the power is changed into a plurality of speeds by the first transmission gearing 32 for transmission to the running device 3. The first transmission gearing 32 provides higher rotational rates than the second transmission gearing 33. The one-way clutch 34 is mounted in the transmission to enable the different rotational rates. When, for example, the vehicle travels down an inclined terrain with the hydraulic clutch 31 disengaged, an accelerating force due to gravity is transmitted through the running device 3 to the transmission shaft 35. With this transmission, however, the one-way clutch 34, which is constructed to allow the higher rotational rates than those transmitted from the second transmission gearing 33, will allow the transmission shaft 35 to be accelerated by such a force. As a result, the vehicle will be accelerated without limitation. Thus, there is room for improvement from the safety point of view.
Among the transmissions being developed recently is one shown in U.S. Pat. No. 4,487,085, which steplessly changes the speed of power with a combination of a cam, a roller, a one-way clutch and the like. As disclosed in the above patent, the one-way clutch is connected in series to a propelling line. Such a transmission, as does the transmission shown in FIG. 5, has room for improvement from the safety point of view since the vehicle will be accelerated while traveling down an inclined terrain.
It is conceivable to provide a transmission mechanism comprising a worm gear between the transmission and running device to prevent acceleration of the running device. However, a transmission having such a worm gear not only entails a substantial power loss during a normal run, but results in enlargement of the transmission per se. This construction too has room for improvement.
It is to be noted that, in a transmission having a worm gear, the worm gear is meshed with a wheel gear, and use is made of the feature that the wheel gear is rotatable by rotation of the worm gear but not vice versa.